This invention relates to lasers and more particularly an improved short pulse laser.
Optical ranging systems operative between the earth and satellites require narrow output pulses from laser transmitters in order to provide the desired ranging accuracy. Such ranging systems may be used, for example, to detect and measure movements of the earth's crust on opposite sides of an earthquake fault. It is desirable that the laser apparatus used in such a laser ranging system be relatively compact to enable direct attachment thereof to the transmitter head and thereby avoid Coude' path optics and design required for a remotely located laser.
A prior laser design utilizes a polarization analyzer and a cavity dump crystal in a Q-switched laser cavity, the crystal being biased at 5-7 kV which is discharged rapidly to divert the light beam from the laser cavity. More specifically, as the biasing voltage rapidly decreases across the dump crystal, an output pulse is generated by the polarization analyzer, the width of the pulse being directly proportional to the rate of change of voltage on the crystal. In order to provide the desired short pulse, the rate of change of voltage must exceed 10.sup.12 volts per second. Since this rate of change exceeds that obtainable with semiconductor devices, gas and spark gap switches are the only known devices capable of operating at the necessary voltage rate. Such switches are relatively bulky and costly compared to semiconductor devices and in addition require that the laser be remote from the transmitter head in a laser ranging system.
In one prior art optically triggered solid state switch, trigger photons are absorbed at the active region of a semiconductor body and generate therein charge carriers which rapidly create a direct current to close the switch. This switch is described in an article entitled "High-Power Switching With Picosecond Precision" by Mourou and Knox, Applied Physics Letters, 35 (7) Oct. 1979, pages 492-495. This optical switch utilizes both silicon and gallium arsenide, and is reported to have operated at switching speeds of 6.times.10.sup.13 to 10.sup.14 volts per second with a holdoff voltage of 5 to 8 kV. In order to use such a switch efficiently, however, it is necessary to have an equally short optical pulse for a trigger. Although such a source exists, it is complex and inconvenient. Further, its use would obviate the need for the laser itself.